1. Field of the Invention
The present invention relates generally to processes for converting sulfide ion to innocuous, water soluble sulfur species, and more particularly, but not by way of limitation, to a process for oxidizing sulfide ion present in solutions containing hydrogen sulfide to sulfate ion.
2. Description of the Prior Art
Sulfide ion is a naturally occurring species that can be a serious problem in many mining, metallurgical and chemical operations. The ion increases corrosivity of water and can be an environmental hazard. It is particularly a problem in the petroleum industry.
When oil is first produced from a subterranean oil formation, there is usually sufficient pressure to force the oil to at least the bottom of the well bore so that it can be pumped out. After a period of time, the pressure sometimes decreases and is no longer sufficient to displace the oil from the reservoir. Due to the fact that a large portion of oil often still remains in the formation after the primary production period has ended, a number of enhanced oil recovery techniques have been developed.
Flooding the formation with water and/or other aqueous solutions is a common secondary recovery or enhanced oil recovery technique. The water is injected into the formation by way of one or more injection wells to drive the remaining oil toward one or more production wells. The oil and water are then pumped out of the production well or wells. In some areas, the only available water for enhanced oil recovery projects must be produced directly from the oil fields. Unfortunately, oil field produced waters often contain a high content of hydrogen sulfide. Water contaminated with sulfide ion, commonly referred to as sour water, is very undesirable. The contaminated water can be highly toxic by inhalation, and is a strong irritant to eyes and mucous membranes. It can also be very corrosive. The bad attributes of sour water make it undesirable for use in flooding operations, and make it very difficult to dispose of.
There are chemicals that are sometimes added to injection water, particularly biocides, that can react with the hydrogen sulfide to form elemental sulfur particles that are difficult to handle. The hydrogen sulfide in the water may react with pipeline steel to form finely divided ferrous particles. Water containing elemental sulfur and/or ferrous particles can plug the reservoir face if it is injected to the reservoir prior to being treated.
Regardless of whether sour water is contaminated by sulfide ion from the reservoir or contaminated by sulfide ion prior to injection in the reservoir, it is difficult to dispose of. The disposal difficulties are due to the environmental and health hazards associated with the water. Permission from governmental agencies for sour water disposal is difficult to obtain. Recycling sour water increases corrosion inhibitor treatment requirements and increases the potential for chemical incompatibility problems.
In order to solve or lessen the above described problems associated with sour water, the toxic sulfide ion can be removed or converted into a water soluble, relatively safe species.
Various processes exist for removing sulfide ion from sour water. For example, hydrogen peroxide or chlorine may be used to oxidize the sulfide ion to elemental sulfur. These processes are suitable for systems that can handle elemental sulfur as an end product. In many applications, however, the precipitated sulfur is not desirable. This is particularly true in closed systems that may become blocked. The precipitated sulfur is colloidal in size and is difficult to filter. The market for elemental sulfur is primarily confined to industrial areas where the sulfur is converted to sulfuric acid.
Thus, there is a need for a process for converting sulfide ion present in aqueous solutions to noncorrosive, innocuous, water soluble sulfur species.